Mud flow drilling rigs are well known, in which drilling fluid is pumped under pressure into a well, via a central wellbore, in order to provide lubrication and cleaning of the well during drilling, and to carry away rock and drill debris from the drill heads.
Referring to FIG. 1 of the drawings, a typical drilling rig comprises a swivel 100, a Kelly 101, a drill pipe 102, a surface casing 103, a wellbore 104 and a wellbore annulus 105. A drilling fluid flow path is defined by an inlet path and a mud flow return line 106. The drilling fluid starts at a storage pit 107, and is drawn up into a suction line 108 by one or more pumps 109, before being fed via a hose 110 to the Kelly 101 and forced down the central wellbore 104 to the drill heads 111. The drilling fluid then flows back up from the drill heads 111, via the wellbore annulus 105 and, as it rises, it flows into the mud flow return line 106. The weight of the drilling fluid is controlled in order to provide downward hydrostatic pressure in the well. In addition, the flow from the annulus is protected by a BOP (Blow Out Preventer) 114, which shuts the well in the event of a sudden outrush of mud flow, known as a kick. The mud flow return line 106 is a gravity fed line designed to give access to the return mud flow for analysis as it flows back into the processing unit 112. The processing unit 112 consists of a series of shakers and filters and a degasser and this filtering process, along with chemical additives, is used to prepare the drilling fluid for reinjection back into the well.
It is desirable to monitor the physical properties and mass flow rate of the drilling fluid, so as to enable maximum efficiency and even operability of the well drilling rig, and aspects of the present invention seek to provide a metering apparatus and method for measuring the balance of the mass flow of drilling fluid going into a well with the drilling fluid flowing back out of the well, via the mud return line, while optionally also offering a metering platform to give other essential measurements such as mud density, sonic velocity and viscosity, as may be required by particular applications. In addition, it would be desirable to provide a means for early kick detection, to give advance warning of a potential “Blow Out”, in order to activate the BOP ahead of any danger.